I was having the same issue, luckily for me however I want to dynamically create tooltips from images, so in the following manner I'm able to set minWidth to the width of the image, hope it might help somebody:

Sadly my "solution" does not work completely in Chrome (need to try Safari too). The tooltip renders with the full width of the image *the first time* it is moused over, but not on subsequent mouse-overs when it reverts to some default.

4.2.1 still has a minor issue with tooltip widths that only affects Firefox on Mac. The following override makes FF/Mac behave. Make sure to remove this override when you upgrade because the fix will be included in the next 4.2.x release.

Code:

Ext.define('Ext.SubPixelRoundingFix', {
override: 'Ext.dom.Element',
getWidth: function(contentWidth, preciseWidth) {
var me = this,
dom = me.dom,
hidden = me.isStyle('display', 'none'),
rect, width, floating;
if (hidden) {
return 0;
}
// Gecko will in some cases report an offsetWidth that is actually less than the width of the
// text contents, because it measures fonts with sub-pixel precision but rounds the calculated
// value down. Using getBoundingClientRect instead of offsetWidth allows us to get the precise
// subpixel measurements so we can force them to always be rounded up. See
// https://bugzilla.mozilla.org/show_bug.cgi?id=458617
// Rounding up ensures that the width includes the full width of the text contents.
if (Ext.supports.BoundingClientRect) {
rect = dom.getBoundingClientRect();
// IE9 is the only browser that supports getBoundingClientRect() and
// uses a filter to rotate the element vertically. When a filter
// is used to rotate the element, the getHeight/getWidth functions
// are not inverted (see setVertical).
width = (me.vertical && !Ext.isIE9 && !Ext.supports.RotatedBoundingClientRect) ?
(rect.bottom - rect.top) : (rect.right - rect.left);
width = preciseWidth ? width : Math.ceil(width);
} else {
width = dom.offsetWidth;
}
// IE9/10 Direct2D dimension rounding bug: https://sencha.jira.com/browse/EXTJSIV-603
// there is no need make adjustments for this bug when the element is vertically
// rotated because the width of a vertical element is its rotated height
if (Ext.supports.Direct2DBug && !me.vertical) {
// get the fractional portion of the sub-pixel precision width of the element's text contents
floating = me.adjustDirect2DDimension('width');
if (preciseWidth) {
width += floating;
}
// IE9 also measures fonts with sub-pixel precision, but unlike Gecko, instead of rounding the offsetWidth down,
// it rounds to the nearest integer. This means that in order to ensure that the width includes the full
// width of the text contents we need to increment the width by 1 only if the fractional portion is less than 0.5
else if (floating > 0 && floating < 0.5) {
width++;
}
}
if (contentWidth) {
width -= me.getBorderWidth("lr") + me.getPadding("lr");
}
return (width < 0) ? 0 : width;
}
});

4.2.1 still has a minor issue with tooltip widths that only affects Firefox on Mac. The following override makes FF/Mac behave. Make sure to remove this override when you upgrade because the fix will be included in the next 4.2.x release.